Transmission photomasks (reticles) used in the production of semiconductor devices are often constructed using conductive metallic films (for example, chromium), or other films, such as MoSiON, deposited onto a transparent substrate, such as quartz. A pattern is etched into this film and then is projected by photo-reduction onto a semiconductor wafer coated with a photosensitive layer. By this means, a replica of the pattern on the reticle is produced in the film on the semiconductor wafer, which replica is greatly reduced in size. Through further and repeated processing of the wafer, a three-dimensional microcircuit is built up.
Such reticles may contain a multitude of isolated conductive features supported on an electrically insulating substrate. These conductive features, which together make up the pattern that is to be projected onto the wafer, can have differing electrical potentials induced on them if the reticle is placed into an electric field. The presence of differing electric potentials on neighboring conductive features can cause electrical discharge between the features in a process that is referred to as field-induced electrostatic discharge (ESD). Furthermore, the features may also be damaged by such induced potentials even when an electrostatic discharge does not take place in a process referred to as electric field-induced material migration (EFM).
The degree of damage that a reticle will suffer as a result of such exposure to an electric field is difficult to predict, since the induction process is dependent upon the detailed structure of the pattern on the reticle, its orientation with respect to the electric field, and its proximity to surrounding objects which might perturb the electric field and concentrate such field through certain areas of the pattern. This makes it difficult to define how frequently a reticle should be inspected for damage in normal use to prevent production of defective wafers. Furthermore, any electric field-induced damage that is sustained by a reticle maybe subtle, highly localized, and difficult to detect during routine reticle inspections. Even though the damage may not be detected in the reticle inspection tool, it may affect the lithographic process.
The damage to the reticle may cause the image projected on the wafer to deviate from that which is expected and which is required for correct functioning of the finished semiconductor device. This is referred to as Critical Dimension (CD) deviation. When a reticle becomes damaged in such a way, defective devices can be produced; and this may not be discovered until the complete device has been built and is tested. Discovery of defects at this late stage in the production process results in significant financial losses to the semiconductor industry.
Electrostatic damage to reticles has been such a prevalent factor in semiconductor production for many years that various novel approaches have been suggested for countering it. In 1984, U.S. Pat. No. 4,440,841 described one of the first methods for making a reticle with an integral conductive layer capable of dissipating electrostatic charge. In 1985, JP Patent No. 60,222,856 described a means of connecting the various mask elements with filamentary conductive lines to avoid potential differences between them. Since those first two approaches, many variants involving conductive coatings, featuring interconnects, and charge dissipating structures have been proposed (e.g., JP Patent No. 62,293,244 (1987); U.S. Pat. No. 5,798,192 (1998); U.S. Pat. No. 5,989,754 (1999); KR Patent No. 196,585Y (2000); U.S. Pat. No. 6,180,291 (2001); TW Patent No. 441,071 (2001); KR Patent Publication No. 2001/057347 (2001); U.S. Pat. No. 6,291,114 (2001); U.S. Pat. No. 6,309,781 (2001); JP Patent Publication No. 2002/055438 (2002); US Patent Publication No. 2002/0115001(2002); U.S. Pat. No. 6,440,617 (2002); U.S. Pat. No. 6,569,576 B1 (2003); TW Patent No. 543,178 (2003); KR Patent Publication No. 2003/085946 (2003); JP Patent Publication No. 2004/061884 (2004); US Patent Publication No. 2004/076834 (2004); and U.S. Pat. No. 6,803,156 (2004)). These solutions increase the complexity and cost of reticle manufacture, plus they add process steps which can introduce defects or inhomogeneity to the reticle. Coatings may delaminate, or they may be easily damaged during handling and reticle cleaning. Furthermore, some of the coatings that have been suggested may degrade due to exposure to energetic UV light that is used in today's leading edge lithography systems; hence, their transparency may alter with time. All of these potential problems probably explain why such solutions are not in widespread use today and why reticle ESD damage continues to be a problem in the semiconductor manufacturing industry.
If the reticle itself cannot be made inherently ESD protected, an alternative solution is to enclose the reticle inside a conductive container, which will provide ESD protection by shielding the reticle from electric fields. Such a solution is described in PCT Publication No. WO 2004/032208. This will protect the reticle while it is inside the container; but semiconductor manufacturing requires the reticle to be moved outside the container on many occasions, during which time the reticle might be exposed to electric fields. Since electric field exposure during normal use of the reticle may gradually change the image on the reticle in a way that is detrimental to the final device that is being manufactured, it is important to be able to monitor a reticle's exposure to electric fields.
US Patent Publication No. 2003/0052691 describes a portable, compact sensor device that is capable of detecting the ESD events in a semiconductor manufacturing facility through their radio emissions. This has been suggested as a means of detecting ESD events in reticles by placing a sensor in the reticle handling environment or in/on the reticle carrier. However, such RF pulse sensing devices can only report the ESD event after the reticle is damaged, and EFM cannot be detected since there is no electrical discharge event. They also are likely to be sensitive to false alarms, owing to the highly charged nature of a semiconductor manufacturing facility.
A more effective and reliable means is required for routinely sensing whether a reticle has been exposed to an electric field. Such a sensor that could warn of a hazardous exposure before the reticle itself becomes damaged would be very desirable.